<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=
    , initial-scale=1.0">
    <title>渐进式图片的替代方案</title>
</head>
<body>
    <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
    <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
    <img src="default.jpg" data-src="http://ww1.sinaimg.cn/large/006y8mN6gw1fa7kaed2hpj30sg0l9q54.jpg" alt="">
    <img src="default.jpg" data-src="http://ww1.sinaimg.cn/large/006y8mN6gw1fa7kaed2hpj30sg0l9q54.jpg" alt="">
    <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
    <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
    <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
    <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
    <img src="default.jpg" data-src="http://ww1.sinaimg.cn/large/006y8mN6gw1fa7kaed2hpj30sg0l9q54.jpg" alt="">
    <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
    <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">


    <script>
        var num = document.getElementsByTagName('img').length;
        var img = document.getElementsByTagName("img");
        var n = 0; //存储图片加载到的位置，避免每次都从第一张图片开始遍历
        lazyload(); //页面载入完毕加载可是区域内的图片
    
        window.onscroll = lazyload;
    
        function lazyload() { //监听页面滚动事件
            var seeHeight = document.documentElement.clientHeight; //可见区域高度
            var scrollTop = document.documentElement.scrollTop || document.body.scrollTop; //滚动条距离顶部高度
            for (var i = n; i < num; i++) {
                if (img[i].offsetTop < seeHeight + scrollTop) {
                    if (img[i].getAttribute("src") == "default.jpg") {
                        img[i].src = img[i].getAttribute("data-src");
                    }
                    n = i + 1;
                }
            }
        }
    </script>

</body>
</html>